1. Field of the Invention
The invention relates to a method of fabricating a silicon-on-insulator substrate including an insulator and a silicon active layer formed on the insulator, and more particularly to such a method including a hydrogen ion separation process. The invention also relates to a silicon-on-insulator substrate suitable for a hydrogen ion separation process.
2. Description of the Related Art
A silicon-on-insulator (hereinafter, referred to as "SOI") structure including a silicon active layer formed on an insulator is considered promising as a substrate to be used for next generation large scale integration (LSI). There have been suggested various methods of fabricating an SOI substrate.
One of such various methods is a method having the steps of forming an oxide film on a surface of a silicon substrate, implanting hydrogen ions into the silicon substrate, overlapping the silicon substrate to a support substrate, and applying heat treatment to the thus overlapped silicon and support substrates to thereby separate the silicon substrate into two pieces at a region to which hydrogen ions have been implanted (hereinafter, this method is referred to as "hydrogen ion separation process").
Hereinbelow is explained the above-mentioned hydrogen ion separation process with reference to FIGS. 1A to 1F.
First, as illustrated in FIG. 1A, there are prepared a silicon substrate 1 and a support substrate 2. A silicon wafer having (100) plane or a plane slightly inclined relative to (100) plane as a principal plane is usually selected as the silicon substrate 1. The same silicon wafer as just mentioned is usually also selected as the support substrate 2.
Then, as illustrated in FIG. 1B, a silicon dioxide film 3 is formed at a surface of the silicon substrate 1. This silicon dioxide film 3 will make an insulating film in an SOI structure. Hence, the silicon dioxide film 3 is designed to have a thickness equal to a thickness of a buried oxide film required for fabrication of a device on an SOI substrate.
Then, as illustrated in FIG. 1C, hydrogen is ion-implanted into the silicon substrate 1 through the silicon dioxide film 3. The thus ion-implanted hydrogen 4 stays in the silicon substrate 1 at a certain depth. When the silicon substrate 1 is subject to heat treatment in a later step, the silicon substrate 1 is separated at that depth into two pieces. One of the two pieces to which the silicon dioxide film 3 belongs makes an SOI active layer in an SOI structure. Hence, in this step of ion-implanting hydrogen into the silicon substrate 1, acceleration energy is controlled for the SOI active layer to have a desired thickness. The silicon substrate 1 is usually implanted at about 30-200 KeV with doses of 1.times.10.sup.16 -3.times.10.sup.17 H.sup.+ cm.sup.-2. The implanted hydrogen ions break bondings between silicon atoms in silicon crystal, and terminate non-bonded hands of silicon atoms.
Then, as illustrated in FIG. 1D, the silicon substrate 1 is laid on top of the support substrate 2 so that surfaces of them make direct contact with each other. Thereafter, the thus overlapped silicon substrate 1 and support substrate 2 are subject to heat treatment.
The heat treatment has two stages.
In a first stage, heat treatment to be carried out at a relatively low temperature in the range of 300 to 800 degrees centigrade is applied to the overlapped silicon substrate 1 and support substrate 2. By carrying out the first stage, the silicon substrate 1 and the support substrate 2 make close contact with each other, and at the same time, the silicon substrate 1 is separated into two pieces at the depth at which hydrogen 4 have been ion-implanted, as illustrated in FIG. 1E.
Hydrogen 4 having been ion-implanted into the silicon substrate 1 in the step illustrated in FIG. 1C is agglomerated at (111) plane or at (100) plane which is parallel to a surface of the silicon substrate 1, as a temperature raises in the first stage of the heat treatment, to thereby form cavities in the silicon substrate 1. If the support substrate 2 is not laid on the silicon substrate 1, a surface layer of the silicon substrate 1 would be peeled off by pressure of hydrogen gas generated in the first stage heat treatment carried out at 300-800 degrees centigrade.
However, in accordance with the hydrogen ion separation process, since the support substrate 2 makes close contact with the silicon substrate 1 with the silicon dioxide film 3 being sandwiched therebetween, the silicon substrate 1 is separated into two pieces one of which remains non-separated from the silicon dioxide film 3 and the support substrate 2. One of the two pieces, which remains on the silicon dioxide film 3, acts as an SOI active layer 5. Thus, there is formed an SOI structure including the support substrate 2, the silicon dioxide film 3 located on the support substrate 2, and the SOI active layer 5 formed on the silicon dioxide film 3. As mentioned above, the SOI active layer 5 is one of the two pieces of the silicon substrate 1.
The separation of the silicon substrate 1 into two pieces is considered partially because of force of deformation caused due to a difference in a thermal expansion coefficient between the support substrate 2 and the silicon dioxide film 3.
Then, in a second stage of the heat treatment, the SOI structure including the SOI active layer 5, the silicon dioxide film 3, and the support substrate 2 is subject to heat treatment at a relatively high temperature, specifically, at 1000 degrees centigrade or greater. Thus, as illustrated in FIG. 1F, there is completed an SOI substrate.
The second stage heat treatment is carried out for the purpose of enhancing bonding force between the support substrate 2 and the silicon dioxide film 3, because it would be impossible to ensure sufficient bonding force therebetween only by the first stage heat treatment.
In the specification, a silicon wafer is distinctive from a silicon substrate. Specifically, the term "silicon wafer" is used as a generic name for indicating a wafer manufactured by a Czochralski (CZ) process, for instance, whereas the term "silicon substrate" is used to indicate a substrate on which an active layer is to be formed in fabrication of an SOI substrate.
When fabrication of an SOI substrate by hydrogen ion separation process is repeated, the other of the two pieces of the silicon substrate 1, removed away in the above-mentioned first stage heat treatment, may be re-used as the silicon substrate 1 or as the support substrate 2 in next fabrication of an SOI substrate.
For instance, Japanese Unexamined Patent Publications Nos. 2-46770 and 9-22993 have suggested fabrication of an SOI substrate by such a hydrogen ion separation process as mentioned above.
Apart from those Publications, fabrication of an SOI substrate by a hydrogen ion separation process has been reported in (a) C. Maleville et al., Silicon-on-Insulator and Devices VII, pp. 34, Electrochem. Soc., Pennington, 1996, and (b) Abe et al., Applied Physics, Vol. 66, No. 11, pp. 1220, 1997.
The hydrogen ion separation process for fabrication of an SOI substrate, having been explained so far, has many advantages as follows, for instance, in comparison with other processes for fabrication of an SOI substrate.
First, it is possible to control a thickness of an SOI active layer, since a thickness of an SOI active layer is dependent on a range distance of ion-implanted hydrogen. The hydrogen ion separation process is suitable in particular for fabrication of a super-thin film SOI substrate.
Second, it is possible to uniformize a thickness of an SOI active layer, and an SOI substrate can readily have a large diameter.
Third, it is possible to reduce fabrication cost, because the hydrogen ion separation process is comprised of steps of ion-implantation and heat treatment both of which are compatible with an ordinary LSI fabrication process.
Fourth, great designability is ensured for thicknesses of an SOI active layer and a buried oxide film.
Fifth, an efficiency for using a wafer is higher than that of other SOI substrate fabrication processes. Specifically, an SOI substrate fabrication process such as a process including the steps of laying a first substrate on a second substrate, and polishing the first or second substrate to thereby make a thin film requires preparation of two wafers for fabrication of an SOI substrate. On the other hand, the hydrogen ion separation process requires preparation of only one wafer for fabrication of an SOI substrate by using a silicon wafer as a support substrate, and re-using a removed piece of a silicon substrate as a silicon or support substrate in next fabrication of an SOI substrate.
However, the hydrogen ion separation process is accompanied with a problem that a resultant SOI active layer includes a lot of crystal defects therein. An SOI active layer made in accordance with the conventional hydrogen ion separation process usually includes crystal defects by the number of about 1.times.10.sup.3 to 1.times.10.sup.4 /cm.sup.2. Such a lot of crystal defects exert harmful influence on characteristics of a device to be formed on an SOI active layer including the crystal defects.
From the standpoint of practical use, it is absolutely necessary to reduce a crystal defect density in an SOI active layer down to about 1.times.10/cm.sup.2 or smaller in order to use an SOI substrate in next generation LSI.
However, not only a method of reducing crystal defects, but also the reason why crystal defects are generated have not been found so far.